Medial habenula (mHb) neurons that project to the interpeduncular nucleus (IPn) regulate the ?set-point? for nicotine aversion and play a critical role in regulating nicotine intake. Nevertheless, little is known about the consequences of nicotine consumption on the function of this circuit. We have collected compelling preliminary data in rodents showing that nicotine reduces habenular volume, induces loss of mHb cholinergic neurons, and triggers degeneration of projection fibers from mHb to IPn. We detect similar reductions of habenular volume in human cigarette smokers compared with non-smokers. Selective lesion of the habenula-IPn circuit markedly increases nicotine intake in rats, suggesting that nicotine-induced damage to the habenula-IPn circuit is likely to increase the motivational significance of the drug. Uniquely, neurons in the mHb express interleukin-18 (IL- 18), a cytokine that regulates the function of microglia and other components of the innate immune system. Microglia are known to control the strength of excitatory and inhibitory transmission by pruning neurons and synapses to sculpt circuits in the brain. We find that nicotine has greater stimulatory effects on the habenula- IPn circuit of Il18-/- mice than wild-type mice. Preliminary findings suggest that nicotine induces far greater damage to mHb neurons in Il18-/- mice than wild-type littermates. Finally, Il18-/- mice demonstrate lower numbers of microglia and reduced levels of microglia-derived cytokines in the habenula than wild-type mice. Based on these findings, we hypothesize that nicotine triggers habenular degeneration and that IL-18, produced by habenular neurons, protects against this response. We further hypothesize that IL-18 acts by recruiting local microglia to prune habenular neurons, thereby opposing the excitatory actions of nicotine on these cells. Here, we will use state-of-the-art molecular, cellular and behavioral approaches to test these hypotheses. In Specific Aim I, we will use iDISCO tissue clearing coupled with light-sheet microscopy for super 3D resolution to fully characterize the role for IL-18 and microglia in regulating excitotoxic effects of nicotine on mHb neurons. In Specific Aim II, we will investigate the role for which IL-18 and microglia in regulating the stimulatory actions of nicotine on mHb neurons. In Specific Aim III, we will investigate the relevance of nicotine- induced damage to the mHb, and the involvement of IL-18 and microglia in this process, to the motivational properties of nicotine. This highly innovative program of research may yield fundamental new insights into the links between brain immune function, habenular plasticity and nicotine addiction.